In recent years, in a mobile communication system (such as a Personal Handyphone System: hereinafter abbreviated as PHS) which is being rapidly developed, to efficiency of utilization of frequencies of radio waves, PDMA (Path Division Multiple Access) in which, by spatially dividing the same time slot of the same frequency, mobile terminal devices of a plurality of users can be multi-path connected to a radio base system has been proposed.
FIG. 6 is a channel allocation diagram in various communication systems of frequency division multiple access (FDMA), time division multiple access (TDMA) and path division multiple access (PDMA).
Referring to FIG. 6, FDMA, TDMA and PDMA will be briefly described. FIG. 6(a) shows the FDMA. Analog signals of users 1 to 4 are frequency-divided and transmitted on radio waves of different frequencies f1 to f4 and signals of users 1 to 4 are split by frequency filters.
In the TDMA shown in FIG. 6(b), digitized signals of respective users are split by time-division on a predetermined time (time slot) unit basis and sent over radio waves of different frequencies f1 to f4 and, signals of respective user are split by a frequency filter and time synchronization among a base station and mobile terminal devices of the users.
In contrast, in the PDMA method, as shown in FIG. 6(c), one time slot of the same frequency is also spatially divided to transmit data of a plurality of users. The number of path-multiplexing and the number of time-division multiplexing shown in FIG. 6(c) are illustrative. According to the configuration of a system, the values are changed.
Specifically, the PDMA (Path Division Multiple Access) method is a method of generating a directivity pattern for each of a plurality of mobile terminal devices in different directions by using an adaptive array device having directivity for both transmission and reception, thereby performing multiplexing communication by sharing carrier waves of one frequency at the same time.
The adaptive array device is constructed by a plurality of antennas fixedly mounted and has a feature in that directivity of the plurality of antennas as a whole is created by adjusting the amplitude and the phase of a transmission/reception signal of each of the antennas.
A radio base station for connecting a plurality of mobile terminal devices by path multiplexing are constructed by using adaptive array devices. The radio base station calculates a weight factor as an adjusting amount of the amplitude and phase with respect to a reception signal per antenna with respect to each mobile terminal device in order to split signals of the mobile stations from the reception signals on which communications of the plurality of mobile terminal devices are multiplexed.
The radio base station compares a signal obtained by multiplexing a reception signal of each antenna with a weight factor of the corresponding antenna and adding the resultant values with a reference signal and adjusts the weight factor so as to minimize the difference between the resultant signal and the reference signal. As the reference signal (unique word signal: UW signal), signal waveform data corresponding to a known fixed bit pattern included in a signal of a control channel or traffic channel is used.
By repeating the adjustment, the radio base station calculates a weight factor for each antenna with respect to each mobile terminal device as an object of path multiplexing, multiples a reception signal of each antenna on which signals of a plurality of mobile stations are multiplexed with the weight factor, and adding the resultant values, thereby separating the signal of each mobile station. At the time of transmission, a signal obtained by multiplying a transmission signal with a weight factor calculated at the time of reception on the antenna unit basis is sent from each antenna, thereby generating a directivity pattern in the mobile terminal device direction.
The path multiplexing technique is written in “Path Division Multiple Access (PDMA) Mobile Communication Method” (IEICE Technical Report RCS93-84 (1994-01), pp 37-44).
A radio base station determines whether communication by path multiplexing is proper or not with respect to a mobile terminal device which is already performing communication and another mobile terminal device newly requesting allocation of a communication channel as follows.
The radio base station calculates response vectors of the two mobile terminal devices as described above, and calculates a correlation value between the calculated two response vectors. The response vector is information regarding the direction in which a mobile station exists, and the correlation value of response vectors is an index indicating closeness of the directions of the two mobile stations.
When the calculated correlation value is larger than the threshold value, the two mobile stations exist almost in the same direction, so that it is impossible to separate the signals of the mobile stations from each other on the basis of the difference between the directivity patterns. In this case, the radio base station determines that multiplexing is improper between them.
The radio base station measures field intensities of signals from the two mobile terminal devices and calculates the ratio between the measured two field intensities. When the calculated field intensity ratio is larger than the threshold value, even when a best directivity pattern is formed, the intensity ratio between the signals is higher than a gain ratio of an adaptive array device. Consequently, it is determined that the signals cannot be properly separated from each other. In this case, the radio base station determines that path multiplexing is improper for them.
When there is no available time slots used for time-division multiplexing communication at the time of allocating a communication channel to a new mobile terminal device, the radio base station for connecting a plurality of mobile terminal devices by also using the time-division multiplexing method makes determination of whether the path multiplexing is proper or not on the time slot unit basis. In a time slot which is not used by a mobile station determined improper to be subjected to path multiplexing with a new mobile station, a communication channel is allocated by path multiplexing.
In such a manner, the radio base station performs path multiplexing only on a set of mobile terminal devices adapted to path multiplexing communication. Further, during the path multiplexing, the directivity pattern of the adaptive array device is made follow the direction in which each mobile terminal device exists, thereby preventing interference and maintaining proper communication quality.
According to the above-described method, at the time of allocating a communication channel to a new mobile station by path multiplexing, the radio base station determines whether the path multiplexing is proper or not by comparing the correlation value of response vectors of a new mobile terminal device and a mobile terminal device already performing communication or the ratio between field intensity of a signal received from a new mobile terminal device and field intensity ratio of a signal received from a mobile terminal device already performing communication with a corresponding threshold value. For the method of determining whether path multiplexing is proper or not, a threshold of DDR (intensity ratio of desired waves from two users), the difference value of reception timings, or the like can be also used.
However, in the case where a reception error generates in a user already performing communication in a slot on which multiplexing is newly performed, it is expected that sufficient precision of a weight vector generated in an adaptive array process of a radio base cannot be obtained. Therefore, in the case where directivity control of the adaptive array is not sufficient in such a state, interference removing capability deteriorates and, as a result, a gain necessary for multiple communication cannot be obtained. It causes a problem such that the characteristics of a down-link communication from the base station deteriorate.
The present invention has been achieved to solve the problems and an object thereof is to provide a radio base station system, a channel allocating method and a channel allocation program capable of assuring predetermined communication quality between a radio base station and a mobile terminal device by improving precision of determining whether path multiplexing is proper or not.